Latitudinal variations in δ30Si and δ15N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years

The stable sedimentary nitrogen isotope compositions of bulk organic matter (inline-formulaδ15Nbulk) and the silicon isotope composition of diatoms (inline-formulaδ30SiBSi) both mainly reflect the degree of past nutrient utilization by primary producers. However, in ocean areas where anoxic and suboxic conditions prevail, the inline-formulaδ15Nbulk signal ultimately recorded within the sediments is also influenced by water column denitrification, causing an increase in the subsurface inline-formulaδ15N signature of dissolved nitrate (inline-formula $M9inlinescrollmathmlchem{\mathrm{italic \delta }}^{normal 15}{\mathrm{normal NO}}_{normal 3}^{-}$ 42pt17ptsvg-formulamathimg78de53da8c88aaea660dd15d6c4ee037 bg-16-2163-2019-ie00001.svg42pt17ptbg-16-2163-2019-ie00001.png ) upwelled to the surface. Such conditions are found in the oxygen minimum zone off the coast of Peru, where, at present, an increase in subsurface inline-formula $M10inlinescrollmathmlchem{\mathrm{italic \delta }}^{normal 15}{\mathrm{normal NO}}_{normal 3}^{-}$ 42pt17ptsvg-formulamathimge12a20ab809bbccf0d78e64582498caa bg-16-2163-2019-ie00002.svg42pt17ptbg-16-2163-2019-ie00002.png from north to south along the shelf is observed due to ongoing denitrification within the poleward-flowing subsurface waters, while the inline-formulaδ30Si signature of silicic acid (inline-formulaδ30Si(OH)4) at the same time remains unchanged.

Here, we present three new inline-formulaδ30SiBSi records between 11 and 15inline-formula S and compare these to previously published inline-formulaδ30SiBSi and inline-formulaδ15Nbulk records from Peru covering the past 600 years. We present a new approach to calculate past subsurface inline-formula $M17inlinescrollmathmlchem{\mathrm{italic \delta }}^{normal 15}{\mathrm{normal NO}}_{normal 3}^{-}$ 42pt17ptsvg-formulamathimg8cf94238272350616b6ec14545ffa862 bg-16-2163-2019-ie00003.svg42pt17ptbg-16-2163-2019-ie00003.png signatures based on the direct comparison of inline-formulaδ30SiBSi and inline-formulaδ15Nbulk signatures at a latitudinal resolution for different time periods. Our results show that, during the Current Warm Period (CWP, since 1800 CE) and prior short-term arid events, source water inline-formula $M20inlinescrollmathmlchem{\mathrm{italic \delta }}^{normal 15}{\mathrm{normal NO}}_{normal 3}^{-}$ 42pt17ptsvg-formulamathimg17e0f65d3774aa421252924f2e446f4c bg-16-2163-2019-ie00004.svg42pt17ptbg-16-2163-2019-ie00004.png compositions have been close to modern values, increasing southward from 7 to 10 ‰ (between 11 and 15inline-formula S). In contrast, during the Little Ice Age (LIA) we calculate low inline-formula $M22inlinescrollmathmlchem{\mathrm{italic \delta }}^{normal 15}{\mathrm{normal NO}}_{normal 3}^{-}$ 42pt17ptsvg-formulamathimg64191ab8a54e55a5372b5628131c6173 bg-16-2163-2019-ie00005.svg42pt17ptbg-16-2163-2019-ie00005.png values between 6 ‰ and 7.5 ‰. Furthermore, the direct inline-formulaδ30SiBSi versus inline-formulaδ15Nbulk comparison also enables us to relate the short-term variability in both isotope compositions to changes in the ratio of nutrients (inline-formula $M25inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}:\mathrm{normal Si}\left(\mathrm{normal OH}{\right)}_{normal 4}$ 70pt16ptsvg-formulamathimge06482812e86ea850323b27f1fc7e754 bg-16-2163-2019-ie00006.svg70pt16ptbg-16-2163-2019-ie00006.png ) taken up by different dominating phytoplankton groups (diatoms and non-siliceous phytoplankton) under the variable climatic conditions of the past 600 years. Accordingly, we estimate a shift from a inline-formula1:1 (or inline-formula1:2) ratio during the CWP and a inline-formula2:1 (up to inline-formula15:1) ratio during the LIA, associated with a shift from overall high nutrient utilization to inline-formula $M30inlinescrollmathmlchem{\mathrm{normal NO}}_{normal 3}^{-}$ 25pt16ptsvg-formulamathimgcbd0aa2bda73584a7a23dafea6b5761c bg-16-2163-2019-ie00007.svg25pt16ptbg-16-2163-2019-ie00007.png -dominated (and thus non-siliceous phytoplankton) utilization.

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Doering, Kristin / Ehlert, Claudia / Martinez, Philippe / et al: Latitudinal variations in δ30Si and δ15N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years. 2019. Copernicus Publications.

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